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Carcinogenesis Advance Access originally published online on October 6, 2006
Carcinogenesis 2007 28(3):657-664; doi:10.1093/carcin/bgl187
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© The Author 2006. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Association of DNA repair polymorphisms with DNA repair functional outcomes in healthy human subjects

Pavel Vodicka1,*, Rudolf Stetina2, Veronika Polakova1, Elena Tulupova1, Alessio Naccarati1, Ludmila Vodickova1,3, Rajiv Kumar4,5, Monika Hanova1, Barbara Pardini1,6, Jana Slyskova1, Ludovit Musak7, Giuseppe De Palma8, Pavel Soucek3 and Kari Hemminki4,5

1 Institute of Experimental Medicine, Videnska 1083, 14200, Academy of Sciences of the Czech Republic Prague, Czech Republic
2 Purkynje Military Medical Academy, Hradec Kralove Czech Republic
3 Center of Occupational Medicine, National Institute of Public Health Prague, Czech Republic
4 Division of Molecular Genetic Epidemiology, German Cancer Research Center Heidelberg, Germany
5 Department of Biosciences at Novum, Karolinska Institute Huddinge, Sweden
6 Department of Biology, University of Pisa Italy
7 Department of Medical Biology, Jessenius Medical Faculty, Comenius Univesity Martin Slovak Republic
8 Laboratory of Industrial Toxicology, Department of Clinical Medicine, Nephrology and Health Sciences, University of Parma Italy

*To whom correspondence should be addressed. Tel: +420 2 41062694; Fax: +420 2 41062782; Email: pvodicka{at}biomed.cas.cz


   Abstract
Top
 Abstract
Introduction
 Materials and methods
 Results
 Discussion
 References
 
We investigated association between polymorphisms in DNA repair genes and the capacity to repair DNA damage induced by {gamma}-irradiation and by base oxidation in a healthy population. Irradiation-specific DNA repair rates were significantly decreased in individuals with XRCC1 Arg399Gln homozygous variant genotype (0.45 ± 0.47 SSB/109 Da) than in those with wild-type genotype (1.10 ± 0.70 SSB/109 Da, P = 0.0006, Mann–Witney U-test). The capacity to repair oxidative DNA damage was significantly decreased among individuals with hOGG1 Ser326Cys homozygous variant genotype (0.37 ± 0.28 SSB/109 Da) compared to those with wild-type genotype (0.83 ± 0.79 SSB/109 Da, P = 0.008, Mann–Witney U-test). Investigation of genotype combinations showed that the increasing number of variant alleles for both XRCC1 Arg399Gln and APE1 Asn148Glu polymorphisms resulted in a significant decrease of irradiation-specific repair rates (P = 0.008, Kruskal–Wallis test). Irradiation-specific DNA repair rates also decreased with increasing number of variant alleles in XRCC1 Arg399Gln in combination with variant alleles for two other XRCC1 polymorphisms, Arg194Trp and Arg280His (P = 0.002 and P = 0.005, respectively; Kruskal–Wallis test). In a binary combination variant alleles of hOGG1 Ser326Cys and APE1 Asn148Glu polymorphisms were associated with a significant decrease in the capacity to repair DNA oxidative damage (P = 0.018, Kruskal–Wallis test). In summary, XRCC1 Arg399Gln and hOGG1 Ser326Cys polymorphisms seem to exert the predominant modulating effect on irradiation-specific DNA repair capacity and the capacity to repair DNA oxidative damage, respectively.

Abbreviations: BER, base excision DNA repair; SSB, single-strand breaks; SNP, single nucleotide polymorphism; PBL, peripheral blood lymphocytes


   Introduction
Top
 Abstract
 Introduction
Materials and methods
 Results
 Discussion
 References
 
In the recent years, several studies have investigated polymorphisms in DNA repair genes and their possible links to the risk of various cancers. Sequence variants in DNA repair genes are assumed to modulate DNA repair capacity and, therefore, are associated with the altered cancer risk. As an example, the hOGG1 Cys/Cys genotype has been associated with an increased lung cancer risk (1). In tobacco-related cancers a protective effect of XRCC1 Arg194Trp variant allele was shown, while variant allele in XRCC1 Arg399Gln polymorphism was associated with an increased risk among light smokers only (2). An increased risk of colorectal cancer was recently reported for XRCC1 Arg399Gln variant allele (3). Statistically significant associations have been found between XPD polymorphisms and skin, breast and lung cancers [reviewed by ref. (4)]. Increasing number of studies relating genetic polymorphisms in DNA repair genes and various kinds of cancer in the past 5 years do not provide unambiguous consistent associations, mainly due to low statistical power for detecting a moderate effect, false-positive results, heterogeneity across study populations (5), failure to consider effect modifiers such as environmental exposures (6) and, most importantly, due to the virtually unknown relationship between the genotype and the functional outcome (phenotype) (7).

An analysis of SNPs in 88 DNA repair genes and their functional evaluation, based on the conservation of amino acids among the protein family members, shows that ~30% of variants of DNA repair proteins are likely to affect substantially the protein function. It applies particularly for polymorphisms in XRCC1 Arg280His and Arg399Gln, and XRCC3 Thr241Met (8). Susceptibility towards ionizing radiation, as measured by prolonged cell cycle G2 delay, was determined in relation to XRCC1 Arg194Trp, Arg399Gln and APE1 Asn148Glu genotypes. Ionizing radiation sensitivity was significantly affected by amino acid substitution variants in both XRCC1 and APE1 genes (9). Using the cytogenetic challenge assay, XRCC1 399Gln and XRCC3 241Met alleles were associated with significant increase in chromosomal deletions as compared with the corresponding homozygous wild-types. Authors concluded that XRCC1 399Gln and XRCC3 241Met are significantly defective in base excision repair (BER), while XPD 312Asn and XPD 751Gln are significantly defective in nucleotide excision repair (NER) (10). Individuals with the wild-type Arg/Arg genotype in XRCC1 Arg194Trp polymorphism exhibited significantly higher values of chromosomal breaks, as assessed by the mutagen sensitivity assay, than those with variant Trp allele, suggesting a protective effect of this allele. On the other hand, variant Gln allele in XRCC1 Arg399Gln was significantly associated with an increase in chromosomal breaks per cell. These data are biologically plausible, since codon 399 is located within the BRCA1 C-terminus functional domain and codon 194 is in the linker region of the XRCC1 N-terminal functional domain (11). Three studies using different approaches have found a functional impact of hOGG1 Ser326Cys polymorphism (12–14), but other studies [reviewed by ref. (1)] did not find any conclusive result for hOGG1 genetic polymorphisms. hOGG1 Ser326Cys polymorphism has also been described to affect the glycosylase function due to the localization and phosphorylation status (15). The results of such tests allow a more meaningful choice of genes for association studies, though they are still not sufficient for an accurate prediction for the DNA repair capacity.

In the present report we attempt to investigate associations between DNA repair genetic polymorphisms (XPD Lys751Gln, XPG Asn1104His, XPC Lys939Gln, XRCC1 Arg194Trp, Arg280His and Arg399Gln, APE1 Asn148Glu, hOGG1 Ser326Cys XRCC3 Thr241Met and NBS1 Glu185Gln) and individual DNA repair activity in a general healthy population from the Central Europe, assessing in vitro the capacity to repair both irradiation-specific induced- and oxidative-induced DNA damage. In the former case, the comet assay (single cell alkaline gel electrophoresis) has been modified to measure the ability of lymphocytes to repair {gamma}-irradiation induced single-strand breaks (SSBs) after 40 min of incubation (16), and in the latter, to measure the ability of a subcellular extract of lymphocytes to carry out the initial incision step of repair on a DNA substrate carrying specific lesions-namely, oxidized bases introduced by visible light in the presence of photosensitizer (17).


   Materials and methods
Top
 Abstract
 Introduction
 Materials and methods
Results
 Discussion
 References
 
Study population
The study was conducted on 244 healthy individuals (183 men and 61 women, mean age 41.3 ± 11.3 years, 90 individuals were smokers and 154 non-smokers) employed in local administration, medical centers and various branches of plastic industry. The investigated population was recruited in the regions of western Slovakia and eastern Bohemia, which exhibit close similarities in socio-economical conditions. Confounding factors, like X-rays, medical drug treatments, dietary (vitamins intake, particular diets) and lifestyle habits (smoking, alcohol and coffee consumptions) and possible exposure-related effects were recorded in detailed questionnaires and considered in the statistical analyses. Present cohort is representative, ethnically homogenous population and therefore suitable for the determination of relationships between DNA repair genetic polymorphisms and DNA repair rates. Lower number of observations for DNA repair rates, in comparison to that given in Table I, were due to methodological limitations (i.e. successful processing of the fresh material). The study design was approved by the local Ethical Committee and the participants provided their informed consent to be included in the study. The sampling of blood was carried out according to the Helsinki Declaration.


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  		Table I Details on investigated SNPs in DNA repair genes

 
DNA repair polymorphisms
Single nucleotide polymorphisms (SNPs) in genes encoding DNA repair enzymes were determined by a PCR–RFLP based method. PCR products were generated using 50 ng of genomic DNA in 25 µl volume reactions containing 20 mM Tris–HCl, 50 mM KCl, 2.0 mM MgCl2, 0.3 mM each dNTP, 0.3 µM each primer (Table I) and 0.2 U Taq DNA polymerase. The temperature conditions for PCR were established as denaturation at 94°C for 30 s, annealing (given in Table I) for 30 s, elongation at 72°C for 30 s and final extension at 72°C for 5 min. The amplified fragments were digested with appropriate restriction endonucleases (Table I) and analyzed. The digested PCR products were resolved on 3% agarose gels containing ethidium bromide and visualized under UV light. The genotype results were regularly confirmed by re-genotyping (10% of samples) and by TaqMan allelic discrimination assay (Assay-on-Demand', Applied Biosystems, Foster City, USA), using Real-Time Gene Amp PCR system on AB 7500 equipment (Applied Biosystems, Foster City, USA). The concordance rate was 100%.

{gamma}-irradiation DNA repair test
Peripheral blood lymphocytes (PBL), isolated using Ficol gradient, were used to test individual DNA repair capacity as described previously (18,19). Briefly, cells embedded in agarose on slides were irradiated with 5 Gy of {gamma}-rays (0.42 Gy/min) and either lysed immediately or incubated at 37°C for 40 min before the lysis. The DNA breaks induced by {gamma}-rays are repaired during the 40 min of incubation period, according to the individual repair capacity. The results (i.e. the amount of repaired SSBs) are calculated as a difference between the initial levels of SSBs, measured immediately after irradiation, and the levels of SSBs detected after 40 min of incubation. The repaired DNA damage is subsequently expressed as SSB/109 Da. Consequently, higher values of repaired SSBs reflect higher DNA repair activity. The detailed description of the tentative origin {gamma}-ray-induced DNA damage as well as the calibration and optimization of the repair test have already been described in details elsewhere (17).

Oxidative DNA repair test
The repair capacity of PBL extracts towards repairing 8-oxoguanine was determined as previously described (20). Briefly, isolated lymphocytes from each individual were collected and divided into aliquots and stored in liquid nitrogen at –80°C, until experiment. Before an assay, a frozen aliquot was thawed and washed with 1% Triton X-100 in a lysis buffer (45 mM HEPES, 0.4 M KCl, 1 mM EDTA, 0.1 mM dithiothreitol, 10% glycerol, pH 7.8) and the lysate was centrifuged to remove nuclei and cell debris. The supernatant was mixed with a reaction buffer (45 mM HEPES, 0.25 mM EDTA, 2% glycerol, 0.3 mg/ml BSA pH 7.8) and kept on ice until use.

A substrate of HeLa cells (2 x 105 per dish) was prepared and pretreated with 2 ml 0.1 µM phosphosensitizer Ro 19-8022 (Hoffmann-La Roche, Basel, Switzerland) and PBSG, and irradiated with a fluorescent lamp (2 min on ice from a 1000 W tungsten halogen lamp, to induce 8-oxoguanines). HeLa cells were successively washed, removed from dishes by gentle trypsinization and embedded in agarose on slides and placed in lysis solution (2.5 M NaCl, 0.1 M Na2EDTA, 10 mM Tris made to pH 10 with NaOH, and 1% Triton X-100) for 1 h at 4°C. After lysis slides are incubated either with individual PBL extracts or with buffer alone at 37°C for 45 min, followed by electrophoresis and neutralization according to comet assay standard protocol (20).

The results (i.e. the amount of repaired oxidative DNA damage, reflecting the removal of 8-oxoguanines) are calculated as a difference between the levels of SSBs, measured in slides with PBL extract and the levels of SSBs measured in slides with buffer only. The level of SSBs, is expressed as SSB/109 Da.

Statistical analyses
Statistical calculations were performed using Statgraphics, version 7 (Manugistics Inc., Cambridge, MA). Hardy–Weinberg equilibrium was tested using the chi-square ‘goodness-of-fit’ test. The data for both DNA repair assays, given in Tables II–VI, are expressed as mean ± SD. For testing significant differences between groups, specifically Table II, the non-parametrical Mann–Whitney U-test was applied. Associations between the combined genotypes and DNA repair rates were tested by Kruskal–Wallis test (as shown in Tables III–VI). Simple linear regression analysis was used to estimate the correlation between confounder and DNA repair rates, whereas multifactorial regression analysis was applied to discern the major influencing factors on the DNA repair rates (i.e. analyzing main confounding factors and DNA repair polymorphisms simultaneously).


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  		Table II Irradiation-specific DNA repair rates and oxidative DNA damage repair rates stratified for individual DNA repair polymorphisms

 


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  		Table III Effect of selected binary combinations of SNPs of BER genes on irradiation-specific DNA repair rates (expressed as SSBs/109 Da)

 
For statistical analyses non-smokers as well as males were assigned as ‘0’, while smokers and females as ‘1’, age was calculated as continuous variable. Similarly, for statistical analyses the wild-type genotype was assigned as ‘0’, heterozygous variant allele bearers as ‘1’ and homozygous variant allele bearers as ‘2’.

Evaluation of DNA repair rates in relation to gene–gene interactions, when three and more polymorphisms were considered, was based on the construction of arbitrary score for variant allele. Wild-type allele was assigned as ‘0’, heterozygous variant allele as ‘1’ and homozygous variant allele as ‘2’. The above approach does not discriminate all possible genotype combinations (i.e. 27 theoretically possible outcomes in ternary and 81 in quaternary combinations), but takes into account a number of variant alleles in particular combination and the higher score reflects the higher number of variant alleles in either genes.


   Results
Top
 Abstract
 Introduction
 Materials and methods
 Results
Discussion
 References
 
Genotype distribution
The genotype distributions for individual DNA repair genes are shown in Table I. Allelic frequencies in XPD Lys751Gln, XPG Asn1104His, XPC Lys939Gln, XRCC1 Arg399Gln, XRCC3 Thr241Met, NBS1 Glu185Gln and APE1 Asn148Glu are in agreement with those earlier described for the central European population (21,22), while allelic frequencies in hOGG1 Ser326Cys, XRCC1 Arg194Trp and Arg280His for the same population are shown for the first time (Table I). The genotype distribution for all investigated polymorphisms, except for XPD Lys751Gln ({chi}2 = 7.0, P = 0.01), was in the Hardy–Weinberg equilibrium.

DNA repair rates and confounders
Both irradiation-specific DNA repair rates and the capacity to repair of oxidative DNA damage were not affected by age and there was no significant difference in both DNA repair rates between men and women. Irradiation-specific DNA repair rates were significantly higher among smokers (1.05 ± 0.81 SSB/109 Da) as compared to non-smokers (0.77 ± 0.62 SSB/109 Da, P = 0.014, Mann–Whitney U-test), while the capacity to repair of oxidative DNA damage was not affected by smoking habit. By investigating simultaneous influence of genotypes in DNA repair and recorded confounders (age, sex, exposure status and smoking), irradiation-specific DNA repair rates were mainly affected by polymorphism in XRCC1 Arg399Gln gene (t = –4.54, P < 0.001), and also by smoking (t = 2.92, P = 0.004, R2 = 0.132; multiple regression analysis). Figure 1 shows the lowest irradiation-specific DNA repair rates being associated with homozygous variant AA XRCC1 Arg399Gln genotype both in smokers and non-smokers, although only in non-smokers the difference in comparison to wild-type GG genotype was statistically significant.


Figure 1
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  		Fig. 1 The {gamma}-irradiation DNA repair rates (expressed as SSB/109 Da) in individuals stratified for smoking habit (smokers n = 80, non-smokers n = 134) and for XRCC1 Arg399Gln polymorphism. The results are presented as mean ± SD, statistical comparison was performed by Kruskal–Wallis test.

 
DNA repair rates and genotype analyses
Irradiation-specific DNA repair rates were significantly decreased in individuals with the homozygous variant (AA) in XRCC1 Arg399Gln than those with the wild-type (GG) and heterozygous (GA) genotypes, (Mann–Whitney U-test: P = 0.0006 and P = 0.002, respectively; Table II).

We did not observe any significant influence on irradiation-specific DNA repair rates in the XRCC1 Arg194Trp and Arg280His and APE1 Asn148Glu polymorphisms. Similarly, no association between the genetic polymorphism in hOGG1 Ser326Cys and irradiation-specific DNA repair rates was observed (Table II).

Our results did not show any effect on irradiation-specific DNA repair rates by genetic polymorphisms in genes involved either in NER (XPD Lys751Gln, XPG Asn1104His and XPC Lys939Gln) or DNA recombination repair (XRCC3 Thr241Met and NBS1 Glu185Gln) (Table II).

Combinations of different polymorphisms in BER genes were investigated in relation to irradiation-specific DNA repair rates. By testing all genotype combinations of XRCC1 Arg399Gln and APE1 Asn148Glu the irradiation-specific repair rates significantly decreased with increasing number of variant (A) allele in XRCC1 Arg399Gln, whereas APE1 Asn148Glu genotype contributed moderately (Kruskal–Wallis test: {chi}2 = 20.87, P = 0.008, Table III).

A significant decrease in irradiation-specific DNA repair rates was also constantly observed in association with variant allele (A) in XRCC1 Arg399Gln, whereas no contribution of the two other investigated polymorphisms in XRCC1 gene (Arg280His and Arg194Trp) was observed [Kruskal–Wallis test: {chi}2 = 14.91, P = 0.002 and {chi}2 = 15.00, P = 0.005, respectively; (Table III, B and C)].

When XRCC1 Arg399Gln polymorphism was not taken into consideration, binary genotype combinations in BER genes did not significantly affect the level of irradiation-specific DNA repair rates (data not shown).

Similar results were observed for combinations of 3 and 4 polymorphisms in BER genes assessed using a score system that reflects the number of variant alleles in particular combination (the data are shown in Table V), revealing again that the predominant effect on the irradiation-specific DNA repair rates is associated with variant (A) allele in XRCC1 Arg399Gln, and this significant tendency persists in spite of increasing number of genes analyzed in combination.


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  		Table IV Effect of a selected binary combination of SNPs of BER genes on the capacity to repair oxidative DNA damage (expressed as SSBs/109 Da)

 


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  		Table V Irradiation-specific DNA repair rates in relation to combinations of SNPs in BER genesa

 
The capacities to repair oxidative DNA damage were significantly decreased in individuals with the homozygous variant (GG) genotype in hOGG1 Ser326Cys as compared to those with wild-type (CC) and heterozygous (CG) genotypes (Mann–Whitney U-test: P = 0.008 and P = 0.041, respectively; Table II).

A significant decrease in the capacity to repair DNA oxidative damage was also associated with combination of variant alleles in hOGG1 Ser326Cys and APE1 Asn148Glu, (Kruskal–Wallis test: {chi}2 = 8.84, P = 0.018, Table IV). As evident from Table IV, the predominant effect is due to the variant G allele in hOGG1 Ser326Cys.

Binary combinations of polymorphisms in hOGG1 Ser326Cys and XRCC1 Arg194Trp, and Arg280His showed that the predominance of the variant G allele in hOGG1 Ser326Cys is associated with the lower capacity to repair DNA oxidative damage (Tables IV and IV).

By testing the effect of all analyzed polymorphisms in BER genes, the capacity to repair DNA oxidative damage decreased with increasing number of variant alleles in hOGG1 Ser326Cys in combination with the increasing number of variant alleles in the other investigated polymorphisms (Kruskal–Wallis test: {chi}2 = 11.07, P = 0.050, Table VI).


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  		Table VI The capacity to repair oxidative DNA damage in relation to combinations of SNPs in BER genesa

 

   Discussion
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
References
 
Age-related decrease has been observed for hOGG1 activity in PBL from healthy individuals (13) as well as for irradiation-specific repair rates (16), whereas both irradiation-specific DNA repair rates and the capacity for the repair of oxidative DNA damage were not affected by age in our study. Interestingly, irradiation-specific DNA but not oxidative DNA damage-related DNA repair rates were significantly higher in smokers than non-smokers. In previous studies xenobiotic exposure-related increase in BER capacities has been recorded in individuals occupationally exposed to styrene (16) and xenobiotics in the tire plant, suggesting possible induction of DNA repair (23). Approximately 2-fold higher irradiation-specific DNA repair rates were found in smokers than in non-smokers (23). These findings may have be consequential, since exposure to potentially carcinogenic industrial chemicals as well as to the complex mixture of carcinogens in cigarette smoke seems to result in an increased BER capacity in healthy, cancer-free population. Whether this increase is due to an induction, or to a process of adaptation, remains to be clarified.

Irradiation-specific DNA repair rates were significantly higher among individuals with the wild-type genotype in XRCC1 Arg399Gln as compared to those with homozygous variant genotype. Because most of the DNA damage induced by {gamma}-irradiation is repaired in a short time (<1 h), the measured DNA repair activity is attributable mainly to the BER pathway (16,18), in agreement with the role of the XRCC1 gene. An observation of the decreased DNA repair capacity in individuals bearing variant A allele in XRCC1 exon 10 (codon 399) is additionally supported by the cytogenetic challenge assay (10), protein conservation analysis (8) and by increased irradiation sensitivity (9). These data seem to be biologically plausible, as XRCC1 protein acts as a coordinator of single strand break repair proteins in the base excision repair pathway with polymorphic codon 399 located within the BRCA1 C-terminus functional domain (11). By testing the effect of other genetic polymorphisms in individual genes involved in BER, i.e. XRCC1 Arg194Trp and Arg280His and APE1 Asn148Glu, we did not observe any significant influence on irradiation-specific DNA repair rates. Although the highest DNA repair rate was seen in just one individual with homozygous variant genotype in XRCC1 Arg280His, no conclusion may be drawn on the base of our present study. On the contrary, irradiation hypersensitivity was observed in 135 women with homozygous variant Glu/Glu genotype in APE1 (9). It becomes more apparent in the light of the occurrence of the variant allele in the general population, which slightly exceeds 3%. The functional significance of XRCC1 Arg280His polymorphism is not yet known (2). The data from the literature indicate that individuals with the wild-type Arg/Arg genotype in XRCC1 Arg280His exhibit significantly higher chromosomal breaks per cell than those with variant His allele. We did not observe any association of polymorphism in codon 194 in XRCC1 Arg194Trp, probably due to low occurrence of the variant allele in our studied population (29 individuals with at least one variant allele). The lack of observed effect of APE1 polymorphism on BER is in agreement with the outcome of computational functional test, which suggested that this SNP is unlikely to exhibit an effect on the protein function (8).

A significant decrease in irradiation-specific DNA repair rates was apparently associated with the binary combination of variant alleles in XRCC1 Arg399Gln and APE1 Asn148Glu polymorphisms. Although polymorphism in APE1 Asn148Glu has no significant effect on irradiation-specific DNA repair, it seems to augment the effect exerted by XRCC1 Arg399Gln polymorphism. An effect of various BER gene polymorphisms in combination on irradiation-specific DNA repair rates was also tested using an attributed score, reflecting a number of variant alleles in the particular combination, since all existing allele combinations could not be tested due to the low frequency of variant allele, particularly in XRCC1 Arg194Trp and Arg280His. Apparently, the highest irradiation-specific DNA repair rates were associated with the lowest score, i.e. with the predominance of wild-type alleles in particular combinations. The results suggest that the main effect is due to the XRCC1 Arg399Gln variant allele.

The capacity to repair oxidative DNA damage was 2-fold higher among individuals with the wild-type genotype (CC) in hOGG1 Ser326Cys as compared to those with homozygous variant genotype. Although the larger functional studies also suggest reduced repair function with variant alleles in hOGG1 (13,24), the evidence is generally inconclusive. On the other hand, variant G allele in hOGG1 Ser326Cys was suggested to affect the glycosylase function due to the localization and phosphorylation status (15). Our data on the hOGG1 Ser326Cys polymorphism and the capacity to repair oxidative DNA damage may provide more quantitative data on the decrease of oxidative damage repair in association with the variant allele in the above gene.

A significant decrease in the capacity to repair DNA oxidative damage was also associated with variant alleles in hOGG1 Ser326Cys and APE1 Asn148Glu polymorphisms, when this binary gene–gene interaction was investigated. Our data suggest that APE1 Asn148Glu polymorphism contributes to highlight an effect of variant G allele in hOGG1 Ser326Cys, although APE1 Asn148Glu polymorphism itself did not influence the oxidative DNA damage repair capacity.

Binary combinations of polymorphisms in hOGG1 Ser326Cys and XRCC1 Arg194Trp, and Arg280His showed that the predominance of wild-type C allele in hOGG1 Ser326Cys is associated with the higher capacity to repair DNA oxidative damage. The proper investigation of gene–gene interactions should be based on substantially larger population and the present data should be cautiously interpreted. Additionally, some other polymorphisms, such as those involved in nucleotide excision repair, may modulate levels of DNA damage as well as activity of OGG1 repair enzyme [higher activity was reported to be associated with the wild-type A allele in XPA gene, (25)].

By investigating simultaneous influence of genotypes in genes coding for BER enzymes and recorded confounders (age, sex, exposure status and smoking), irradiation-specific DNA repair rates were significantly affected by polymorphism in XRCC1 Arg399Gln and by smoking. These data suggest the importance of gene–environment interactions and the research in this direction should be continued. Similarly, the capacity to repair DNA oxidative damage was significantly modulated by tentative exposure status and by hOGG1 Ser326Cys polymorphism. A participation of environmental and occupational exposure factors on both irradiation-specific DNA repair rates as well as on the capacity to repair oxidative DNA damage has been reported earlier, suggesting that the particular DNA repair pathways may be induced by the exposure to xenobiotics (16,23).

An understanding of the relationships between DNA repair polymorphisms and corresponding functional reflections may contribute to the interpretation of results obtained from case–control association studies on various types of cancer. In order to clarify the roles of DNA repair polymorphisms and DNA repair capacities, as important susceptibility factors affecting the onset of cancer, both markers need to be analyzed first in general healthy population (background levels) and subsequently compared with those found in newly diagnosed, untreated cancer patients.


   Acknowledgments
 
The study was supported by grants EU Diephy FOOD-CT-2003-505609, IGA MZ NR8563-5/2005, GACR 310/05/2626 and by AVOZ 50390512.

Conflict of Interest Statement: None declared.


   References
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 

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Received June 22, 2006; revised September 25, 2006; accepted September 27, 2006.


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